High-density glass fibre granules

ABSTRACT

A method for preparing glass strand pellets by stirring chopped glass strands in the presence of 10 to 25% of water, the strands being sized with a size containing an organosilane. The method performs stirring for long enough for an increase in density to be at least 67%, the stirring being performed using a single stirring equipment that at each instant imparts to the strands or forming pellets it contains the same stirring frequency, the pellets finally formed containing, after drying, at least 95 wt % glass. A film-forming agent is in contact with the glass strands during the stirring at the latest.

The invention relates to the preparation of glass strand pellets bystirring glass strands. The glass strands concerned can be used toreinforce polymer-based thermoplastics, more commonly known as RTPs (forreinforced thermoplastics) and known in French as TPAs, for“thermoplastique armé”. The thermoplastics concerned are especiallypolyolefins such as polyethylene or polypropylene, polyamides orpolybutylene terephthalate.

The manufacture of fiber-reinforced thermoplastics using chopped glassstrands passes through a phase of compounding and mixing a thermoplasticpolymer and chopped glass strands in an extruder. This preparation isperformed at a high enough temperature for the polymer to be fluidenough and for the final reinforced thermoplastic composition to be asuniform as possible. Specifically, the presence of clumps of strands inthe thermoplastic generally results in inferior mechanical properties(particularly in terms of impact strength) and/or a degraded surfacefinish.

In general, the extruder performs the following functions:

-   -   it filamentizes (that is to say breaks down) the clumps of glass        strands,    -   it compounds the glass strands with the thermoplastic matrix as        uniformly as possible,    -   it heats the glass strands/thermoplastic compound to a        temperature above the softening temperature of the        thermoplastic, then produces an extruded bead of glass        strands/thermoplastic compound, it being possible for said        extruded bead to be cut up to turn it into pellets.

The chopped (glass) strand is usually in the form of an assembly of manyindividual filaments. These strands form an integral whole, possiblycontaining, for example, from 10 to 4000 filaments. The filaments mayhave a diameter ranging from 5 to 24 μm, for example about 10 μm orabout 14 μm.

In order to make the glass strands easier to handle, attempts are madeat agglomerating them, for example into the form of clumps such aspellets or granules. Indeed, such clumps are easier to handle and tometer out than traditional chopped strands. In addition, these clumpshave a higher apparent bulk density and the same mass of glass strandstherefore takes up a smaller amount of volume, something which isequally beneficial from the points of view of storage, of transport andof handling. This density, measured using the standardized ISO 15100method, needs to be high enough to give economic transport costs andeasy and reliable metering on entering the extruder. The term densityused in this application is indeed this apparent density determined byISO 15100.

The clumps (pellets in the case of the present invention) of choppedglass strand need to be intact enough that they do not deteriorate inuse. Indeed, the various mechanical actions (transportation, unwrapping,conveying, metering) may give rise to the creation of “fines” that makethe chopped strand unsuited to correct use. From another standpoint,this intactness must not be too great either because the clumps need tobe opened up (that is to say broken down into individual filaments) atthe appropriate moment and completely when compounded with thethermoplastic pellets in the extruder.

U.S. Pat. No. 4,840,755 describes a vibration method for slightlydensifying the starting strands and making them into rod-like forms. Thewidth with which the strands arrive is practically the same as that withwhich they leave.

WO9640595 (from the same family as U.S. Pat. No. 5,578,535) relates to acomposition comprising pellets obtained by hydration in order to obtaina water content from 11 to 20%, then by compounding the fibers for atleast 3 minutes until pellets are formed, then drying said pellets. Theratio of the densities of the pellets to the starting strands is about1.2 to 1.3.

WO9843920 (of the same family as U.S. Pat. No. 5,868,982 and U.S. Pat.No. 5,945,134) relates to a method for manufacturing pellets involvingthe following successive steps: forming strands comprising a number offilaments, cutting the strands, applying a hydrating solution to thestrands, dispersing the hydrating solution over the strands in a firsttumbling operation in a first zone until pellets are formed, densifyingthe pellets by subjecting them to a second tumbling operation in asecond zone. The pellets thus produced are cylindrical and have adiameter representing 20 to 65% of their length. According to thatdocument, it is impossible to obtain sufficient densification if theoperations of agglomeration (formation of pellets), on the one hand, andof densification, on the other, are not separated, by carrying them outin different apparatuses.

WO0149627 discloses a method for manufacturing pellets comprising thefollowing successive steps: forming strands comprising a number offilaments sized with a first precursor, chopping the strands, applying asolution of a binder containing a copolymer of maleic anhydride andanother copolymerizable monomer, dispersing the solution over thestrands in a first tumbling operation in a first zone until pellets areformed, densifying the pellets by subjecting them to a second tumblingoperation that is not as vigorous as the first in a second zone. Anincrease in density by 13 to 60% by comparison with the initial choppedstrands is thus obtained. It is the existence of a second tumblingoperation that allows greater densification, up to 60% increase over thechopped strands used, to be obtained.

The increase in productivity dictates that very harsh transport methods(for example pneumatic transport) be considered, this increase inproductivity requiring, amongst other things, very high flowabilityproperties in order to guarantee high feed rates and extremely precisemetering.

Usually, the conventional chopped strand has a length of 3 or 4.5 mm,these lengths having been adopted because of the good compromiseachieved between intactness and density. The need to make thiscompromise has hitherto always prevented producers of fibers for RTPfrom considering longer strands (for example 9 or 12 mm strands),because then the transport and metering of such strands are not suitedto the conventional extruders. However, such an increase in length wouldhave the advantage of increasing the residual length in the compositeand therefore of improving the mechanical properties of the endcomposite. Given the developments of extruders to screw profiles capableof keeping the longest possible lengths, it is therefore possible toenvision preparing longer glass strand pellets.

The invention relates to a method for preparing glass strand pellets bystirring chopped glass strands in the presence of 10 to 25 wt % ofwater, said strands being coated with a size containing an organosilane,said method performing stirring for long enough for the increase indensity to be at least 67%, this being performed using stirringequipment that, at each instant, imparts to the strands or equipmentpellets forming, the same stirring frequency, the pellets finally formedcontaining, after drying, at least 95 wt %, or even at least 99 wt % itcontains glass, a sticky (film former) agent being in contact with theglass strands during stirring at the latest.

In the pellets obtained using the invention, the “filaments” are moreclosely packed than in the simple fiber-forming operation using thebushing. The shape factor of the pellets leads to optimum density.

The glass strands used in the context of the invention are generallymanufactured using the following succession of steps:

-   -   drawing the filaments in a damp atmosphere through bushings from        molten glass, then    -   coating the filaments with a sizing liquid, then,    -   gathering the filaments together into strands, then,    -   cutting the strands to form chopped glass strands.

At this stage, the chopped strands are wet. They generally contain from5 to 25 wt % of water, for example 5 to 15 wt % of water. There is noneed to dry them before introducing them into the stirring stepaccording to the invention because this step has in any case to beperformed in the presence of water. Thus, any additional water needed(with respect to the water supplied by the fiber drawing step) is addedto the stirring apparatus in order to achieve a total water content(water due to the fiber drawing including the sizing water plus wateradded to the stirring apparatus) ranging from 10 to 25 wt % andpreferably from 12 to 15 wt % of the mass introduced into the stirringapparatus. It is possible and preferable not to have to add additionalwater (to reduce the soiling of the pelletizer and increase theefficiency). To achieve this, all that is required is for the fiberforming to be performed at sufficient wetness to obtain correctpelletization.

The sizing liquid contains at least one organosilane. This organosilanegenerally contains at least one reactive group capable of reacting withthe hydroxyl groups at the surface of the glass to graft the modified(in that its reactive group has reacted and therefore that it has lostpart of the said reactive group) organosilane to the surface of thefilaments. The organosilane used for the sizing operation is generallythe hydrolyzed derivative of an alkoxysilane, itself generallycontaining the trialkoxysilane group, that is to say —Si(OR)₃, Rrepresenting a hydrocarbon radical such as a methyl or ethyl or propylor butyl radical. The organosilane can therefore for example be thehydrolyzed derivative of one of the following compounds:

-   -   γ-aminopropyltriethoxysilane    -   γ-glycydoxypropyltrimethoxysilane.

The organosilane is generally present in the sizing solution at a rateof 0.05 wt % to 1 wt % and preferably 0.2 to 0.6 wt %. The sizingsolution may also contain other ingredients, such as a film former, alubricant, an antistatic agent. The sizing liquid may be a solution, anemulsion or a suspension.

Following the sizing step, the filaments are assembled into strandsgenerally containing 10 to 4000 filaments, then chopped to the desiredlength. These two steps (assembly and chopping) are known per se tothose skilled in the art. This then yields chopped glass strands sizedwith an organosilane. In general, the chopped strands used contain lessthan 200 ppm (in terms of weight) of fines (comprising from 1 filamentto 10 agglomerated filaments).

The stirring apparatus may be any type of equipment capable of stirringthe compound comprising the chopped strands without damaging them in anyway. In order not to cause the strands in the pellets that are beingformed to break up, the stirring must not to be too vigorous. Thestirring imparts a repetitive movement to the chopped strands, then tothe pellets being formed. The rotation frequency of the equipment mayfor example range from 10 to 50 revolutions per minute.

As a preference, the stirring is a tumbling operation, which means thatthe strands or pellets being formed are lifted up and fall back onthemselves, tumbling, and this continues until the desired pellets areobtained. The pellet must not break up as it tumbles. As a preference,tumbling carries the strands or pellets it contains at a linear speedranging from 0.2 to 1 meter per second and preferably 0.3 to 0.7 meterper second, particularly about 0.5 meter per second. This drive speed isthat of the wall of the equipment coming into contact with the strandsor pellets being formed in order to drive them. This linear speed can berepresented by a vector tangential to the wall driving the strands orpellets, such as the vector v in FIG. 2.

The stirring according to the invention may be achieved in a singlestirring step. This means that there is no need to resort to twodifferent stirring equipment, for example for the start of stirring onthe one hand and the end of stirring on the other. The stirring cantherefore be performed in the one same equipment. In addition, in thecase of a single stirring equipment, it is not necessary to formdifferent stirring zones by modifying, for example, the geometry of theequipment into different zones, the various zones subjecting the pelletsbeing formed to different stirring constraints. Different stirringconstraints might, for example, be more or less vigorous tumbling, thatis to say tumbling at different frequencies. The stirring apparatus cantherefore have just one single stirring zone. From the start to the endof the stirring operation, the chopped strands and the pellets formed orbeing formed may be subjected to the same constraints by the stirringapparatus, particularly since, for example, the stirring frequency isconstant. Thus, the equipment may be such that it affords stirring,particularly tumbling, the frequency of which is identical at everyinstant for its entire contents, that is to say for the chopped strandsor the pellets being formed. In the case of tumbling, the tumblingfrequency is generally higher than the rotational frequency of theequipment (number of revolutions per unit time) because as can be seenin particular from FIG. 9, when the equipment performs one revolution,the objects inside it can tumble on themselves several times. It isconsidered that the equipment drives everything it contains at the samefrequency because all these objects are subjected to the same stirringconstraints. The equipment may also be such that it drives the strandsor pellets being formed at a linear speed that is constant from thestart (chopped-strand stage) to the end of the preparation of thepellets.

The pellets may be prepared continuously by equipment imparting stirringat a constant frequency, from the starting chopped-strand stage rightthrough to the end pellet stage.

The equipment may also contain partitions routing the pellets beingformed so as to limit the mixing between pellets in the only slightlyformed state and pellets in an advanced stage of formation.

The stirring equipment, more particularly the tumbling equipment,generally rotates about an axis and stirs everything it contains (fromthe chopped strands to the pellets) with the same frequency. At everyinstant, the equipment has just one rotational frequency (or radialspeed of rotation). Everything that the equipment contains is stirred atthe same frequency, this frequency generally being higher than thefrequency of tumbling of the objects inside.

A tumbling operation may for example be carried out in a hollow cylinderrotating about its axis of revolution. The cross section of the cylindermay be cylindrical or may have another suitable shape, for example apolygonal, for example hexagonal, shape. The axis of revolution ispreferably inclined to the horizontal by an angle ranging from 0 to 45°.Such a cylinder is depicted in FIG. 1. This cylinder comprises a tubularsurface 1 and an end wall 2. In the variant of FIG. 1, the cylinder isnot very deep (by comparison with its diameter) and could also be termeda plate. This cylinder has its axis of revolution XX′ forming an anglealpha with the horizontal. This cylinder may be made to rotate about itsaxis of revolution by a motor 3. The chopped strands and the otheringredients of the compound are intended to be placed in the cylinder.It can be seen that the strands are tumbled and follow a path of thekind depicted in FIG. 2 with dotted arrows, said figure depicting thecylinder viewed in the direction of its axis of revolution, saidcylinder comprising the tubular surface 1 and the end wall 2. In thisvariant, it is possible either to vary or not to vary the tumblingfrequency during stirring. However, even if the frequency is variedduring stirring, it is obvious that, at every instant, the tumblingfrequency is the same for all the chopped strands and pellets containedin the equipment at the same time.

It is possible to assist the tumbling with hammer blows onto therotating tumbling apparatus (cylinder or plate). FIG. 9 depicts such avariant. The hammer 10 periodically strikes the rotating apparatus 11,encouraging the strands or pellets being formed on the interior wall ofthe apparatus to detach. As a preference, the objects 12 contained inthe apparatus tumble in the portion of angle β of about 90° between avertical line and a horizontal line both passing through the axis ofrotation.

For a continuous industrial process, the cylinder may be a collection ofseveral concentric sub-cylinders fixed one above the other, the pelletspassing from one to the next through orifices. Such an assembly isdepicted in FIG. 4, the path of the pellets being depicted using arrows.In this variant, the pellets pass from an upstream sub-cylinder to adownstream sub-cylinder having spent a certain residence time in theupstream sub-cylinder, and so on. Such circulation, by better separatingthe pellets according to their density, allows the spread on pelletparticle size to be reduced. In addition, the proliferation of pathsmakes it possible to increase the residence time and therefore optimizethe volume of the pelletizer with respect to the mass produced per unittime. The equipment therefore here contains partitions routing thepellets being formed so as to avoid, as much as possible, pellets in anearly stage of formation mixing with pellets in an advanced stage offormation. Such equipment gives the same tumbling frequency (dependenton the rotational frequency of the equipment) to the chopped strandsentering and to the pellets leaving. Even though everything theapparatus contains is tumbled at the same frequency, it is, however,found that the tumbled objects are distributed here over different radiiand that the circumferential speed changes in each stage. It istherefore necessary for the equipment to be dimensioned and operated insuch a way that the objects in the large diameters are not preventedfrom tumbling by centrifugal force and that the objects in the smalldiameters are spun fast enough that they can tumble. This apparatustherefore affords tumbling by way of stirring, the frequency of whichtumbling at every instant is the same for the chopped strands (entering)and the pellets, including pellets being formed and pellets leaving. Inthis variant that can be used for continuous manufacture, the tumblingfrequency is generally kept constant. FIGS. 7 and 8 show plate variants.FIG. 7 shows a spiral plate, a spiral-shaped partition parallel to theaxis of revolution of the plate being secured to its base. The pelletsbeing formed follow the spiral path dictated by the partition. Thepellets are placed in the middle and re-emerge at the periphery. FIG. 8shows a plate comprising a number of concentric partitions parallel tothe axis of rotation of said plate, orifices in said partitions allowingthe pellets being formed to pass from a volume between two partitions toan adjacent volume. This passage from one volume to the next is in thedirection from the center toward the periphery.

Thus, stirring can be performed in a cylinder having the shape of aplate, with a diameter larger than its depth, said plate being equippedwith partitions parallel to the axis of rotation and increasing theresidence time of the pellets. The stirring apparatus receives thechopped strands at the center and the pellets leave via the periphery ofthe plate.

For a continuous industrial process it is also possible to use a hollowcylinder (a tube if the cross section of the cylinder is round) the axisof revolution of which is inclined to the horizontal and which comprisesa long enough tubular surface for the strands being converted intopellets to travel from one end of the cylinder to the other. The crosssection perpendicular to its axis of revolution may be round or have anyother suitable shape, for example be polygonal, for example hexagonal.This cylinder may have a small amount of conicity (for example 5%),converging or diverging. The conicity is defined by the ratio, as apercentage (large diameter−small diameter)/length along the axis. Theprinciple of such a cylinder is depicted in FIG. 3. The cylinder isinclined by an angle alpha to the horizontal 4. The chopped strands areloaded into the cylinder through one of its openings 5, the one at anelevated position by comparison with the other opening, the strandsbeing converted into granules then following a path of the kind depictedin dotted line in FIG. 3, the formed pellets being recovered through theoutlet opening 6, the one in the lowered position by comparison with theinlet opening 5. Such equipment is considered to have just one stirringzone because, from the start to the end of stirring, the chopped strandsfollowed by the pellets being formed are subjected to the same stirringconstraints by the equipment. The cylinder may also be a collection ofseveral concentric sub-cylinders fixed one above the next, the pelletspassing from one to the next through orifices. Such an assembly isdepicted in FIG. 5, the path of the pellets being depicted using arrows.In this variant, the pellets pass from an upstream sub-cylinder to adownstream sub-cylinder having spent a certain residence time in theupstream sub-cylinder, and so on. Such circulation, by producing bettersegregation of the pellets according to their density, makes it possibleto reduce the spread on the particle size of the pellets. The equipmenttherefore here contains partitions routing the pellets being formed inorder to prevent as far as possible pellets at an early stage offormation from mixing with pellets at an advanced stage of formation.Here also, the equipment performs tumbling by way of stirring, thefrequency of which tumbling at every instant is the same for the choppedstrands (entering) and the pellets, including the pellets being formedand the pellets leaving. Here too, the tumbling frequency is generallykept constant and this equipment can also be used for continuousmanufacture. The equipment in FIGS. 3 and 5 are examples for which thelinear speed at which the strands and pellets are driven may be constantthroughout the conversion of the strands into pellets.

The stirring may also be performed in a rotating bicone such as the onedepicted in FIG. 6. This bicone (7), equipped with an opening (8) set inrotation via a shaft (9). The axis of the bicone being able to adopt aninclination θ that can vary according to the operation concerned:Loading with chopped strands θ=45°, adding water θ=0°, unloading at theend of pelletizing θ=90°. By way of example, this bicone can operate ata ° rotational frequency about the shaft 9 of 30 revolutions per minute.

Use may also be made of a pelleting plate, an inclined cylinder open atboth ends, a fixed cylinder where the fibers are set in motion through avortex effect.

Before the stirring operation, the ingredients of the compound to bestirred are introduced into the stirring equipment. There are thereforeintroduced:

-   -   the chopped sized strands, and    -   at least one film-forming agent, and    -   water to represent 10 to 25 wt % of the total mass of said        compound.

The chopped sized strands are generally wet and therefore alreadycontribute some of the 10 to 25% of water needed for the methodaccording to the invention.

The film-forming agent and the water are in contact with the glassstrands during stirring at the latest. This means that the film-formingagent can be brought into contact with the glass strands right from thefiber-forming operation, for example during the sizing by beingintroduced into the sizing liquid, or may be brought into contact withthe glass strands later, independently of the sizing step, byintroducing it separately into the stirring apparatus, generally beforestirring, or possibly during stirring.

The film-forming agent may be introduced at least partially separatelyfrom the chopped strands. However, the film-forming agent may just aseasily be introduced at least partially at the same time as the strandsbecause it is carried by the strands. This is particularly the case ifthe sizing liquid contains film-forming agent. All the film-formingagent needed for the stirring operation can be contributed by thestrands, following its application to the strands during the sizingoperation. In this case, no additional amount of film-forming agent isadded to the fibers after the sizing step.

The film-forming agent may be present in an amount of 0.3 wt % to 2 wt %of the total mass to be stirred. The film-forming agent has the purposeof giving the chopped strand some cohesion (it holds the filamentstogether within the chopped strand). However, the film-forming agentmust not prevent the filaments from separating from one another whenpassed through the extruder. The person skilled in the art knows whichfilm-forming agents can be used.

The film-forming agent may thus be chosen from the following compounds:

-   -   polyester,    -   polyurethane,    -   an epoxy polymer, for example a polymer of diglycidyl ether of        bis-phenol A,    -   an epoxy-polyurethane copolymer.

In particular, use may be made of Neoxil 962 by DSM.

As the person skilled in the art knows, the film-forming agent needs tobe selected according to the nature of the thermoplastic that is to bereinforced. For a thermoplastic of the polyester type, such as PBT orPET, use may be made of a film-forming agent of the epoxy type,particularly of a polymer of diglycidylether of bisphenol A (DGEBA). Fora thermoplastic of the polyamide type, use may be made of a film-formingagent of the polyurethane type.

The water may be introduced into the stirring apparatus at leastpartially separately from the chopped strands. However, the water isgenerally also introduced at least partially at the same time at thestrands because it is contributed by the strands, following the sizingoperation. Specifically, the cut strands are not generally dried beforethe stirring step. All the water needed for the stirring operation mayalso be contributed by the strands, following its application to thestrands particularly during the sizing operation.

If not all of the water needed for the method according to the inventionis contributed by the strands when they are introduced into theequipment, this water can be added directly to the stirring equipment byany suitable means, particularly by spraying/atomizing or by addingsteam. The addition of steam is a preferred way of adding water, whenwater needs to be added directly (without being carried by the strands)to the stirring equipment. This is because it has been found that theuse of steam results in the pellets obtained being more uniform and in ahigher pellet-forming rate.

If water needs to be added to the stirring apparatus independently ofthe chopped strands, it is possible to mix it, before introducing itinto the stirring equipment, with another ingredient, for example atleast some of the film-forming agent. This has an advantage when it isnot desirable to apply the film-forming agent to the strands during thesizing step, for example for reasons of toxicity incompatible with thefiber drawing/sizing operation, or alternatively if the film-formingagent reacts with another ingredient in the sizing composition or isdetrimental to the stability of the sizing emulsion.

The sizing operation may therefore contribute to the surface of thestrands some or all of the amount of film-forming agent and total waterneeded. Typically, according to a preferred embodiment, all the amountof film-forming agent needed is introduced into the liquid with whichthe strands are sized and then no more need be added after the sizingoperation. This is advantageous from the point of view that the entiremethod is simplified, and from the point of view that if a film-formingagent has to be added in a step subsequent to the sizing itself, forexample by spraying, risks inherent to the handling of this kind ofproduct, for example the risks of the spray nozzles becoming blocked,are run. In addition, if, during such a step subsequent to sizing, thereis a desire to add some of the water needed as a mixture with thisfilm-forming agent, it would not be possible to use steam for thisaddition.

The sizing operation necessarily contributes at least some of the waterneeded, if not all of it. In general, water is also added directly tothe stirring apparatus, independently of the strands. As the strandsgenerally contribute water in amounts of 5 to 15 wt % of the total massto be stirred, water is generally added directly to the stirringequipment at levels of 5 to 10 wt % of the total mass to be stirred sothat 10 to 25% and preferably 12 to 15% of the total mass being stirredconsists of water. This is pure water, that is to say containing atleast 99% water.

Thus, according to a preferred variant of the method, the sizecontributes all the film-forming agent and at least some of the water,and topping-up water is simply added directly to the stirring apparatusin the proportions just given. The size is therefore generally“complete” which means that it incorporates all the ingredients of aconventional sizing compound for the envisioned application, and that itis generally unnecessary to add any of these ingredients after sizing,except possibly water.

The residence time that the strands spend in the stirring apparatus inorder to yield pellets is generally at least 2 min, and more generallyat least 4 min, and more generally at least 8 min, for example 10 min.It is possible to stir for longer, but that is unnecessary. Thus,stirring may be achieved in under 15 min. The stirring is performed forlong enough to obtain the desired pellet density.

The stirring is generally performed at ambient temperature.

As a preference, the interior surface of the stirring equipment ishydrophobic. As a preference, the interior surface of the stirringequipment is resistant to abrasion. As a preference, the interiorsurface of the stirring equipment is slippery enough with respect to themoving glass strands. Such properties can be provided by a coating. Thiscoating may be made of a hydrophobic polymer such as PTFE or PVDF. Ithas been found that the moving strands had less of a tendency to stickto the walls if the equipment had an interior surface made of suchmaterials, which results in better efficiency. As a preference, theinterior surface has suitable roughness, for example an Ra value of 1.5.

The chopped strands are clumped together next to each other duringstirring to form the pellets, without modifying their length. Thus, thepellets more or less take on the form of cylinders the lengths of whichare roughly identical to the lengths of the longest strands introducedto start with.

Use may be made of chopped strands with a length ranging from 1.5 to 25mm, particularly from 2 to 25 mm such as 2 to 15 mm and moreparticularly 3 mm, 4.5 mm, 5 mm, 9 mm or 12 mm.

Use may also be made, by way of strands, of a mixture of strands ofdifferent lengths.

The starting chopped strands may also contain fines because these finesplay an important part in the pelletization by clumping together andentering the pellets.

The filaments contained in the strands may have a diameter ranging from5 to 24 μm.

Stirring is performed for long enough to obtain the desired pelletdiameter or the desired increase in density. The method according to theinvention makes it possible to prepare pellets the density of which isat least 35%, or at least 50%, or at least 67%, or at least 80%, or atleast 100%, or at least 130%, or even at least 200% greater than thedensity of the starting chopped strands. In general, maximum density isobtained when the pellet diameter reaches a value roughly equal to itslength.

The method according to the invention makes it possible to obtainpellets having a low loss on ignition (LOI). This stems from the factthat it is possible, in the context of the present invention, to usesmall amounts organic compounds such as the organosilane or thefilm-forming agent. Thus, the pellet according to the invention may havea loss on ignition of less than 0.8% and even less than 0.5%, forexample ranging from 0.1 to 0.5%, particularly ranging from 0.2 to 0.4%.

The final pellet can be defined as an object consisting of the closecontact of many parallel glass filaments with individual diametersranging from 5 to 24 μm, these filaments all having the same nominaldiameter or having different nominal diameters. The number of filamentscontained in a pellet may in particular range from 50 000 to 500 000depending on the diameter of the filaments, for example 360 000 to 500000. The filaments are tightly packed in the pellets. Table 2 belowgives examples of pellets that can be obtained using the methodaccording to the invention: TABLE 2 Filament Diameter diameter Number ofof the end (μm) filaments pellet (mm) 5 5000 0.4 5 500000 3.5 24 500005.4 24 500000 17.0 5 5000 0.4 5 50000 1.1 5 200000 2.2 10 5000 0.7 1050000 2.2 10 200000 4.5 24 5000 1.7 24 50000 5.4 24 200000 10.7

The pellet is generally in roughly cylindrical form, its approximatediameter ranging between 1 and 10 mm. In the case of a few very largepellets compared with the others, under magnification these may looklike they are made up of two or three closely associated cylinders. Inthe case of pellets at least 9 mm long and longer, the cylinder in somecases may be slightly deformed, the filaments not being in contact overtheir entire length but having slipped along their axis, meaning thatthe pellets therefore have a length significantly greater than that ofthe starting chopped strands. For a basic chopped strand length (used tostart with) of 12 mm, the pellets may thus lengthen into a point up to16 mm long. These pellets therefore contain a roughly cylindricalcentral body, the base of each cylinder being extended by a point, likein an olive. Thus, for pellets at least 9 mm long, their length may beat least 10% greater than that of the starting chopped standards andtherefore of the filaments they contain.

The pellets generally have a bulk density at least 67% higher than thebulk density of the starting chopped strands. They generally haveroughly the same length as said starting chopped strands, especiallywhen the length of said pellets is less than 9 mm.

The pellets contain size suited to the reinforcing of thermoplastics,said size generally having been applied to the strands before they werechopped into chopped strands.

There is no need to form a polymer jacket around the pellets toencapsulate them. This is because the pellets produced according to theinvention are intact enough to be used as they are after drying. Theycan therefore be used as they are (dried) to feed into an extruder (orany other suitable compounding machine) which is also fed withthermoplastic (for example PE, PP, PS) generally also in the form ofpellets. The fact that they are not encapsulated means that they arebroken down more readily at the time of their use in order to becompounded with the thermoplastic.

Examples of Batch Pelletization

The bicone of FIG. 6, which has an internal volume of 11.5 liters, isloaded with 2000 g of chopped strands of density “dens” (see table 1).These strands comprising about 800 to 4000 10-μm filaments have beencoated with size during the fiber drawing operation using a conventionalapplicator roller, and a sizing liquid containing an organosilane, thehydrolyzed derivative of γ-aminopropyltriethoxysilane marketed under thereference A1100 by Crompton-OSI and a film-forming agent of the polymerof diglycidyl ether of bis-phenol A type. These strands contain x wt %water (see table 1). Their loss on ignition (LOI) is y wt %. The amountof water needed to obtain the desired moisture content (see table 1) isthen added in the form of steam (“V” in table 1) or by spraying (“P” intable 1). Once the cover has been closed, the bicone is placed in aθ=45° position and the device is set in continuous rotation at the speedof 30 revolutions per minute for 10 minutes.

The main characteristics of these examples are given in table 1(operating conditions and results). This table gives:

-   -   the characteristics of the starting chopped strands, namely:        -   their length “L” in mm,        -   their density “Dens”, measured by the ISO 15100 method,        -   their water content “x” in wt %,        -   their loss on ignition (LOI) “y” in wt %;    -   the way in which the water was added, namely:        -   the means: steam “V” or spraying “P”,        -   the amount of water added as a wt % of the total mass for            stirring,    -   the total water content at the time of stirring,    -   the characteristics of the end pellet, namely:        -   their length “L” in mm,        -   their density “Dens” measured using the ISO 15100 method;    -   the increase in density between the density of the starting        chopped strands and the pellets.        Example of Continuous Pelletization

Pellets are produced using the device depicted in FIG. 10. After fiberdrawing during which the fibers are coated with size, the strands arechopped, the chopped strands then being conveyed to the pelletizationequipment in the form of a tube, the pellets then being conveyed to thedrying and then screening operations, after which the pellets arepackaged.

The manufacturing conditions were as follows: Fiber drawing: Bushing:1200 holes Output: 650 kg/day Filament diameter: 10 μm Chopped strands:Cut length: 4.5 mm Loss on ignition: 0.69% Moisture content when cut:14.5% Pelletization: Pelletizing tube length: 3.30 m Pelletizing tube Ø:240 mm Tube inclination: 1.9° Rotational speed: 40 revolutions perminute Tumbling aid system (hammer): 2 blows per revolution Residencetime: 2 min Drying: Fluidized bed vibrated at: 180° C. Residence time: 2min

The chopped strands are introduced into the moving tube directly withthe correct pelletization moisture content. Table 3 collates theresults: TABLE 3 Mean Number Moisture Increase in pellet of Filament ØFilament content Loss on density by diameter filaments (μm) length (wt%) Density ignition pelletization (mm) per pellet 10 4.5 14.54 0.87 0.6961% 2.6 70000 10 4.5 12.82 0.95 0.60 60% 2.8 80000 10 4.5 12.5 0.8 0.6662% 3.2 100000 10 4.5 13 0.93 0.58 70% 4.5 200000 17 12 13.5 0.71 0.81200% 2.3 17000

TABLE 1 Chopped strands Total Increase in L Added water water Pelletdensity Ex n^(o) (mm) Dens x (%) y (%) Means % (wt %) L (mm) Dens % 14.5 0.53 10 0.7 V 1 11 4.5 0.8 51 2 4.5 0.53 10 0.7 V 2 12 4.5 0.9 70 34.5 0.53 10 0.7 V 3 13 4.5 1 89 4 4.5 0.53 10 0.7 V 4 14 4.5 0.95 80 54.5 0.53 10 0.7 V 8 18 4.5 0.75 41 6 3 0.53 5 0.7 V 7 12 3 0.85 66 7 4.50.53 5 0.7 V 7 12 4.5 0.9 70 8 6 0.3 5 0.7 V 7 12 6 0.85 183 9 12 0.2 50.7 V 7 12 12 0.85 330 10 6 0.3 10 0.7 V 4 14 6 0.8 166 11 12 0.2 10 0.7V 4 14 12 0.8 300 12 9 0.25 10 0.7 P 2 12 9 0.9 260 13 9 0.25 10 0.7 P 414 9 0.85 240 14 4.5 0.43 5 0.4 P 8 13 4.5 0.9 110 15 4.5 0.43 5 0.4 V 813 4.5 0.9 110 16 9 0.25 5 0.4 P 8 13 9 0.9 260 17 9 0.25 5 0.4 V 8 13 90.88 250

1-28. (canceled)
 29. A method for preparing glass strand pelletscomprising: stirring sized chopped glass strands, the strands containingcontiguous glass filaments, in a presence of 10 to 25 wt % of water, thestrands having been coated with a size containing an organosilane, themethod performing the stirring for long enough for an increase indensity to be at least 67%, using a single stirring equipment that ateach instant imparts to the strands or forming pellets it contains asame stirring frequency, the pellets finally formed containing, afterdrying, at least 95 wt % glass, a film-forming agent being in contactwith the glass strands during the stirring at the latest.
 30. The methodas claimed in claim 29, wherein the stirring comprises a tumblingoperation.
 31. The method as claimed in claim 30, wherein the stirringequipment drives the strands or forming pellets it contains at a linearspeed ranging from 0.2 to 1 meter per second.
 32. The method as claimedin claim 31, wherein the stirring equipment drives the strands orforming pellets it contains at a linear speed ranging from 0.3 to 0.7meter per second.
 33. The method as claimed in claim 32, wherein thestirring equipment drives the strands or forming pellets it contains ata linear speed of about 0.5 meter per second.
 34. The method as claimedin claim 30, wherein the tumbling is assisted by striking of a hammer ona tumbling apparatus.
 35. The method as claimed in claim 29, wherein allof the film-forming agent is applied to the strands while they are beingsized.
 36. The method as claimed in claim 29, wherein the film-formingagent is present in an amount representing 0.3 wt % to 2 wt % of a totalmass to be stirred.
 37. The method as claimed in claim 29, wherein thewater is wholly introduced contributed by the chopped strands.
 38. Themethod as claimed in claim 29, wherein the water is introduced into thestirring equipment partly as a contribution from the strands, and partlyintroduced directly into the equipment independently of the strands. 39.The method as claimed in claim 38, wherein the water contributed by thestrands represents 5 to 15 wt % of a mass to be stirred and the waterintroduced directly into the equipment represents 5 to 10 wt % of themass to be stirred.
 40. The method as claimed in claim 38, wherein thewater introduced directly is added in sprayed or atomized form.
 41. Themethod as claimed in claim 29, wherein the strands have a length rangingfrom 1.5 to 15 mm.
 42. The method as claimed in claim 29, wherein thechopped strands contain less than 220 ppm in terms of weight of finescomprising 1 to 10 filaments.
 43. The method as claimed in claim 29,wherein the stirring is performed for long enough to obtain an increasein density of at least 80%.
 44. The method as claimed in claim 43,wherein the stirring is performed for long enough to obtain an increasein density of at least 100%.
 45. The method as claimed in claim 44,wherein the stirring is performed for long enough to obtain an increasein density of at least 130%.
 46. The method as claimed in claim 45,wherein the stirring is performed for long enough to obtain an increasein density of at least 200%.
 47. The method as claimed in claim 29,wherein the granules have a loss on ignition of less than 0.5%.
 48. Themethod as claimed in claim 29, wherein an interior surface of thestirring equipment is covered with a coating made of hydrophobicpolymer.
 49. The method as claimed in claim 29, wherein the stirring isperformed at a constant frequency, from a starting chopped strand to anend pellet.
 50. The method as claimed in claim 29, wherein the stirringis performed in a cylinder having a shape of a plate, with a diameterlarger than its depth, the plate being equipped with partitions parallelto an axis of rotation and increasing a residence time of the pellets.51. The method as claimed in claim 50, wherein the stirring equipmentreceives the chopped strands at a center and the pellets re-emerge by aperiphery of the plate.
 52. The method as claimed in claim 29, whereinthe stirring equipment includes only one stirring zone.
 53. A pelletobtained by the method as claimed in claim
 29. 54. A pellet with adiameter of between 1 and 10 mm, comprising, in close contact, 50,000 to500,000 parallel glass filaments with individual diameters ranging from5 to 24 μm.
 55. The pellet as claimed in claim 54 comprising 360,000 to500,000 glass filaments.
 56. The use of pellets as claimed in claim 54to reinforce a thermoplastic.
 57. The use of pellets as claimed in claim56, wherein the pellet is not encapsulated in a polymer.